The present application is generally related to inground operations and, more particularly, to a system, apparatus and method for electrically coupling an electrical signal onto an electrically conductive drill string for purposes of transferring the signal.
Generally, an inground operation such as, for example, drilling to form a borehole, subsequent reaming of a borehole for purposes of installing a utility line, borehole mapping and the like use an electrically conductive drill string which extends from an above ground drill rig. The prior art includes examples of the use of an electrically conductive drill string as an electrical conductor for serving to electrically conduct a data signal from an inground tool to the drill rig. The surrounding earth itself serves as a signal return path for purposes of detecting the signal at the drill rig. This type of system is often referred to as a Measurement While Drilling, MWD, system.
An example of an attempt to use the drill string as an electrical conductor in an MWD system is seen, for example, in U.S. Pat. No. 4,864,293 (hereinafter, the '293 patent). In one embodiment, the patent teaches an electrically isolated collar that is fitted around the drill string. Applicants recognize that the use of such an electrically isolated collar (FIG. 2, item 32) is problematic at least with respect to durability in what can be an extremely hostile inground environment. In another embodiment, shown in FIGS. 3 and 4, a suitable dielectric separator 40 is diagrammatically shown and asserted to electrically isolate a front section of the drill string from the remainder of the drill string. No detail is provided that would reasonably teach one how to fabricate this separator, but it is reasonable to assume that the isolator would simply be inserted into a break in the drill string for co-rotation therewith. Unfortunately, the isolator would then be subject to the same rigorous mechanical stresses during the drilling operation as the drill pipe sections of the drill string including pure tension force during pullback operations and high shear forces due to rotational torque that is applied to the drill string by the drill rig. While the drill string is generally formed from high strength steel that can readily endure these forces, Applicants are unaware of any currently available non electrically conductive material that is capable of enduring all these different forces with a reliability that Applicants consider as acceptable. It should be appreciated that the consequences of breaking off the end of the drill string during a drilling operation are extremely severe. Thus, the risk introduced through the use of an isolator in the suggested manner is submitted to be unacceptable.
An even earlier approach is seen in U.S. Pat. No. 4,348,672 in which an attempt is made to introduce an electrically isolating break in the drill string using various layers of dielectric material that are interposed between the components of what the patent refers to as an “insulated gap sub assembly” that is made up of first and second annular sub members. One embodiment is illustrated by FIGS. 5 and 6 while another embodiment is illustrated by FIGS. 7 and 8 of the patent. Unfortunately, the practice of interposing relatively thin dielectric layers in a gap defined between adjacent high-strength metal components, that are competent to withstand extreme forces as well as a hostile downhole environment, is unlikely to provide an acceptable level of performance. In particular, these dielectric layers are subjected to the same severe forces as the first and second annular sub members such that durability in a hostile downhole environment is most likely to be limited. That is, the desired electrical isolation will be compromised at the moment that one of the relatively thin dielectric layers is worn through.
Practical approaches with respect to coupling an electrical signal onto a drill string in the context of an MWD system are seen, for example, in U.S. patent application Ser. No. 13/035,774 (hereinafter the '774 application) and U.S. patent application Ser. No. 13/035,833 (hereinafter, the '833 application), each of which is commonly owned with the present application and each of which is incorporated herein by reference in its entirety. The latter applications take the highly effective approach of using a downhole current transformer to inductively couple a downhole signal onto the drill string while still maintaining physical performance characteristics that are comparable to those of the drill string itself. While the '774 and '833 applications provided sweeping advantages over the then-existing state of the art, Applicants have discovered other highly advantageous approaches, as will be described hereinafter.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.